Transthyretin (TTR), (also known as prealbumin, hyperthytoxinemia, dysprealbuminemic, thyroxine; senile systemic amyloidosis, amyloid polyneuropathy, amyloidosis I, PALB; dystransthyretinemic, HST2651; TBPA; dysprealbuminemic euthyroidal hyperthyroxinemia) is a serum/plasma and cerebrospinal fluid protein responsible for the transport of thyroxine and retinol (Sakaki et al, Mol Biol Med. 1989, 6:161-8). Structurally, TTR is a homotetramer; point mutations and misfolding of the protein leads to deposition of amyloid fibrils and is associated with disorders, such as senile systemic amyloidosis (SSA), familial amyloid polyneuropathy (FAP), and familial amyloid cardiopathy (FAC).
TTR is synthesized primarily by the liver and the choroid plexus of the brain and, to a lesser degree, by the retina in humans (Palha, Clin Chem Lab Med, 2002, 40, 1292-1300). Transthyretin that is synthesized in the liver is secreted into the blood, whereas transthyretin originating in the choroid plexus is destined for the CSF. In the choroid plexus, transthyretin synthesis represents about 20% of total local protein synthesis and as much as 25% of the total CSF protein (Dickson et al., J Biol Chem, 1986, 261, 3475-3478).
With the availability of genetic and immunohistochemical diagnostic tests, patients with TTR amyloidosis have been found in many nations worldwide. Recent studies indicate that TTR amyloidosis is not a rare endemic disease as previously thought, and may affect as much as 25% of the elderly population (Tanskanen et al, Ann Med. 2008; 40(3):232-9).
At the biochemical level, TTR was identified as the major protein component in the amyloid deposits of FAP patients (Costa et al, Proc. Natl. Acad. Sci. USA 1978, 75:4499-4503) and later, a substitution of methionine for valine at position 30 of the protein was found to be the most common molecular defect causing the disease (Saraiva et al, J. Clin. Invest. 1984, 74: 104-119). In FAP, widespread systemic extracellular deposition of TTR aggregates and amyloid fibrils occurs throughout the connective tissue, particularly in the peripheral nervous system (Sousa and Saraiva, Prog. Neurobiol. 2003, 71: 385-400). Following TTR deposition, axonal degeneration occurs, starting in the unmyelinated and myelinated fibers of low diameter, and ultimately leading to neuronal loss at ganglionic sites.
The compounds and treatment methods described herein provide significant advantages over the treatments options currently available for TTR related disorders. TTR amyloidosis typically lead to death within ten years, and until recently, was considered incurable. Liver transplantation is an effective means of replacing the disease-associated allele by a wild type (WT) allele in familial cases because the liver is typically the source of amyloidogenic TTR. While liver transplantation is effective as a form of gene therapy it is not without its problems. Transplantation is complicated by the need for invasive surgery for the recipient and the donor, long-term post-transplantation immunosuppressive therapy, a shortage of donors, its high cost, and the large number of TTR amyloidosis patients that are not good candidates because of their disease progression. Unfortunately, cardiac amyloidosis progresses in some familial patients even after liver transplantation because WT TTR often continues to deposit. Central nervous system (CNS) deposition of TTR is also not relieved by transplantation owing to its synthesis by the choroid plexus. Transplantation is not a viable option for the most prevalent TTR disease, senile systemic amyloidosis (SSA), affecting approximately 25% of those over 80 due to the deposition of WT TTR.
Antisense technology is emerging as an effective means for reducing the expression of specific gene products and may therefore prove to be uniquely useful in a number of therapeutic, diagnostic, and research applications for the modulation of TTR expression (See U.S. Patent Publication Nos. 2008/0039418 and 2007/0299027.
The present invention provides compositions and methods for modulating transthyretin expression. Antisense compounds for modulating expression of transthyretin are disclosed in the aforementioned published patent applications. However, there remains a need for additional such compounds.